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POIs my method for avoiding dynamic_cast<> faster than dynamic_cast<> itself?
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copied!<p>I was answering a <a href="https://stackoverflow.com/questions/2758158/which-casting-technique-is-better-for-doing-casting-from-upper-class-to-lower-cla">question</a> a few minutes ago and it raised to me another one:</p> <p>In one of my projects, I do some network message parsing. The messages are in the form of:</p> <pre><code>[1 byte message type][2 bytes payload length][x bytes payload] </code></pre> <p>The format and content of the payload are determined by the message type. I have a class hierarchy, based on a common class <code>Message</code>.</p> <p>To instantiate my messages, i have a static parsing method which gives back a <code>Message*</code> depending on the <strong>message type byte</strong>. Something like:</p> <pre><code>Message* parse(const char* frame) { // This is sample code, in real life I obviously check that the buffer // is not NULL, and the size, and so on. switch(frame[0]) { case 0x01: return new FooMessage(); case 0x02: return new BarMessage(); } // Throw an exception here because the mesage type is unknown. } </code></pre> <p>I sometimes need to access the methods of the subclasses. Since my network message handling <strong>must</strong> be fast, I decived to avoid <code>dynamic_cast<></code> and I added a method to the base <code>Message</code> class that gives back the message type. Depending on this return value, I use a <code>static_cast<></code> to the right child type instead.</p> <p>I did this mainly because I was told once that <code>dynamic_cast<></code> was slow. However, I don't know exactly <strong>what it really does</strong> and how slow it is, thus, my method might be as just as slow (or slower) but far more complicated.</p> <p>What do you guys think of this design ? Is it common ? Is it really faster than using <code>dynamic_cast<></code> ? Any detailed explanation of what happen under the hood when one use <code>dynamic_cast<></code> is welcome !</p> <p><strong>--- EDIT ---</strong></p> <p>Since some people asked why:</p> <p>Basically, when I receive a frame, I do two things:</p> <ol> <li>I parse the message and build a corresponding instance of a subclass of <code>Message</code> if the content of the frame is valid. There is <em>no</em> logic except for the parsing part.</li> <li>I receive a <code>Message</code> and depending on a <code>switch(message->getType())</code>, I <code>static_cast<></code> to the right type and do whatever has to be done with the message.</li> </ol>

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